1. Field of the Invention
This invention is concerned with magnetic bubble devices that include a layer a magnetic material supported on a generally nonmagnetic substrate. Such devices depend on the nucleation and/or propagation of enclosed single wall magnetic domains, having polarization opposite to that of the surrounding material in the supported layer, for their operation. Functions which these devices may perform include memory, switching, logic, etc.
2. Description of the Prior Art
In recent years, there has been interest in a class of magnetic devices known generally as bubble devices. These devices generally use a supported planar layer of material capable of manifesting uniaxial magnetic anisotropy and of supporting single wall magnetic domains, generally called bubbles. Although capable of assuming any one of several different configurations, bubbles are usually cylindrical with the direction of magnetization normal to the surface of the layer and opposite to that of the surrounding material in the layer. Materials capable of supporting bubbles include Co.sub.5 Sm garnets, rare earth orthoferrites and magnetoplumbites.
A bubble is stable in most materials over a range of diameters that typically varies by a factor of three. As bubble size, at any temperature, depends on the magnetic field within the supported layer, a bias field is usually applied to obtain a bubble diameter in the middle of the range. Because the magnitude needed for the bias field depends upon the magnetization of the supported layer, which for most magnetic materials is temperature dependent, the bubble size will vary with temperature unless the bias field varies. If the size variations become extreme, the bubbles will disappear either through radial instability where the magnetic field is strong enough to collapse the bubbles or through elliptical instability where the magnetic field is not strong enough to prevent the bubbles from expanding into long (strip) domains. A temperature dependent range of bias field values in which bubbles are stable is thus defined. Within this range, it is desirable to maintain bubbles at a constant size. Bubbles are usually moved within the layer of supported material by overlay patterns of magnetically soft material and a time varying in plane magnetic field. Size variations may either cause variable coupling between the bubbles and the drive field and overlay patterns or increased domain interaction and lead to erratic propagation.
Although bubble materials can be produced with almost temperature independent characteristics including bubble size, for a temperature independent bias field, in general, the bubble size is a function of temperature unless the bias field has the temperature dependence that maintains the bubbles at constant size. It can be seen that if the bias field does not have the required temperature dependence, i.e., if it does not keep the bubbles at a reasonably constant size, the temperature range in which the bubble material may operate under optimum conditions will be restricted.
Bias magnet materials with a temperature dependent magnetic field that closely matches that needed to maintain a constant size bubble are known and have been used to expand the temperature range over which particular bubble devices are usefully operated. Some exemplary materials are discussed in U.S. Pat. No. 3,711,841 of J. E. Geusic and L. G. VanUitert.
Bubble materials have been recently developed, see Applied Physics Letters, 26 402 (1975), that appear to have significant advantages over prior materials because bubbles may be propagated in them at higher velocities than possible in prior materials before bubble propagation becomes erratic and information is lost. A high bubble velocity is desirable because a major contemplated use of bubbles is in memories in which bubbles are accessed sequentially and a high velocity reduces the time needed to access a particular bubble or absence thereof and retrieve that bit of information. The operating speed of the machine using bubble memories may thus be increased. These materials do, however, require a strongly temperature dependent magnetic bias field, approximately 0.6 percent/degree C. at 50 degrees C. to maintain the bubbles at constant size and permit operation over an extended temperature range.
As the bias field is usually supplied by a permanent magnet, the material used in the magnet should have those properties that are desirable in permanent magnets including, in a preferred form, a high coercive force. The material used for the magnet thus preferably satisfies two requirements if it is to be used advantageously with the bubble materials described above: (1) the temperature dependence of its magnetic field must be large compared to prior materials and (2) it preferably has a high coercive force.